not true, the relationship between OSTs and anime are vague, its like a comparison of your height and your skill in basketball, some animes i like don't even have any soundtrack that gets my attention.
OSTs don't come from the producers themselves, but they get a choice on the pick, the soundtracks usually come from studios that they have come to rely to. (a good example of this i found was in a special "in the making" video of Full Metal Alchemist Conqueror of Shambala movie, a 30 minute video of the journey of the directors that i enjoyed watching.)

Hmm what's a good soundtrack anyway? I'd say that most general long standing serial animes like Naruto have second-rateish to not bad sound tracks, but overall the series is lacking lead-way and drive.

More obscure and quieter animes have unique soundtracks, like Bartender, had decent storyline and animation, but it captured the audience through it's OST and music more often time and ambient relaxing moods it creates.

a great OST would be songs by Hitomi Kuroishi who made music for Code Geass. Two of the songs Stories and Continued stroy is soothing a great to listen to when relaxing.

Well, while their songs were excellent, I think when they meant OST I think they meant like every song ever played within the series. Background, interludes, etc. Though Code Geass had a pretty vast and impressive OST though!